Most of today's advanced semiconductor processes yield complimentary metal-oxide semiconductor (CMOS) having breakdown voltages of no more than approximately 4.0 volts (V). Therefore, if the voltage supplied to such CMOS chips is greater than 4.0 V (e.g. 5.0 V) it is regulated down to an acceptable lower voltage such as 3.3 V. Because many disk-drive systems include a 5.0 V power supply, CMOS chips, such as a disk-drive controller that are used in disk-drive systems often include an on-board voltage regulator.
In a typical disk-drive system, it is common for the 5.0 V supply to temporarily dip below 5.0 V due to power fluctuations and voltage spikes. For example, power fluctuations can be caused by sudden starts and stops of the voice-coil motor (VCM) which moves the read head. If the supply voltage falls below 5.0 V for a specified length of time, then a typical disk-drive controller circuit enters a power-down mode so that the circuits drawing power from the regulator will significantly decrease their current requirements. But if the supply voltage becomes too low, the regulator may be unable to provide a regulated voltage to the power-down circuitry of the disk-drive controller. Unfortunately, this lack of a regulated voltage may cause the power-down circuitry to malfunction. Furthermore, the disk-drive controller may incorrectly interpret temporary dips in the supply voltage as an indication of a power-down condition.